What Hit Jupiter?

August
3, 2009: It began with a furrowed brow, a moment
of puzzlement, quickly dismissed.

The
date was July 19, 2009. Amateur astronomer Anthony Wesley
was photographing Jupiter from his backyard observatory in
Murrumbateman, Australia, when something odd caught his eye.

"My
attention was fixed on the Great Red Spot, which was setting
beautifully over Jupiter's horizon," recalls Wesley.
"I almost didn't notice the dark blemish near Jupiter's
south pole, and when I did, I put it out of my mind."

It's
just another dark storm on Jupiter.

"That's
what I thought at first, but something about the dark mark
puzzled me, it didn't look right, and I couldn't stop stealing
glances at it."

Above:
South is up in this July 19th discovery image taken by Anthony
Wesley using a 14.5-inch telescope in Murrumbateman, Australia.
[more]

Slowly,
Jupiter's rotation turned the blemish toward Earth, Wesley
got a better look at it, and the truth struck him like a thunderbolt.

It
was an impact mark. Something hit the giant planet!

"I
had seen the scars caused by fragments of Comet Shoemaker-Levy
9 hitting Jupiter in 1994, so I knew what an impact looked
like," he says. "After I'd convinced myself that
this was real, I could hardly use the computer. My hands were
shaking. It was quite unbelievable."

He
quickly emailed his photos to friends and colleagues around
the world, and within hours telescopes great and small were
turning toward Jupiter to photograph the aftermath of a powerful
collision.

"We
believe it was a comet or asteroid measuring perhaps a few
hundred meters wide," says Don Yeomans of NASA's Near-Earth
Object Office at JPL. "If something of similar size hit
Earth—we're talking about 2000 megatons of energy--there would
be serious regional devastation or a tsunami if it hit the
ocean."

In
a stroke of luck almost as big as Wesley's, JPL astronomers
Glenn Orton and Leigh Fletcher were already scheduled to observe
Jupiter on July 20th, barely a day after impact, using NASA's
Infra-red Telescope Facility (IRTF) atop Mauna Kea in Hawaii.
The 3-meter telescope revealed a fresh cloud of debris about
the size of Mars floating among Jupiter's clouds.

Right:
An IRTF image of the Jupiter impact debris cloud on July 20,
2009. The cloud appears bright at this wavelength (2.12 microns)
because particles in the cloud are reflecting infrared radiation
from the sun, explains observer Glenn Orton. [more]

"The
object, whatever it was, exploded in Jupiter's upper atmosphere,"
says Orton. "It blew itself to smithereens. What we're
seeing now are bits and pieces of the impactor and possibly
some strange aerosols formed by shock-chemistry during the
impact."

On
July 23rd, the Hubble Space Telescope took its first pictures
of the blast site. Hubble was still undergoing checkout and
calibration following the STS-125 servicing mission in May,
but this event was too big to skip. Space Telescope Science
Institute director Matt Mountain allocated emergency telescope
time to a team of astronomers led by Heidi Hammel of the Space
Science Institute in Boulder, Colorado.

As usual, Hubble photos stole the show. They
revealed a swirling maelstrom of dark cindery debris jostling
with natural storms near the top of Jupiter's atmosphere:

Above:
A Hubble Space Telescope image of the Jupiter impact scar
taken on July 23, 2009, taken using Hubble's new camera, the
Wide Field Camera 3 (WFC3). [more]

Shuttle
astronaut and veteran Hubble repairman John Grunsfeld says
he was delighted by the photo "because it was the first
image to be released from the brand-spanking new WFC-3 camera
that Drew Feustel and I installed in May. Thanks to WFC-3,
we got to see the impact in stunning detail."

"The
debris cloud is lumpy because of atmospheric turbulence,"
explains planetary scientist Amy Simon-Miller of the Goddard
Space Flight Center. "Polar winds blowing 25 m/s (~55
mph) are causing it to spread out and grow larger. This
will make the cloud even easier to see through backyard telescopes."

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Judging
from the behavior of the Comet Shoemaker-Levy 9 impacts fifteen
years ago, she estimates that the 'Wesley debris cloud' could
remain visible for many weeks to come. Researchers will put
the time to good use. Further studies of the cloud might yet
reveal the great unknown:

Indeed, there was no warning. The object emerged
from darkness, unknown and uncatalogued, and—wham!—before
anyone could photograph the body intact, it had become a cloud
of debris. (There is a lesson here for Earth, but that is
another story.)

The cloud's chemical composition holds clues
to the nature of the impactor. Orton says ground-based observers
are now analyzing light reflected from the cloud to figure
out what it is made of. "If the spectra contain signs
of water, that would suggest an icy comet. Otherwise, it's
probably a rocky or metallic asteroid."

Meanwhile, it's a big dark mystery—the kind
that Wesley can't take his eyes off of. "I am still observing
Jupiter almost every night using my 14.5 inch telescope,"
he says. "The cloud is expanding and taking on some interesting
shapes."

How
do we know the size of the impactor? To explain,
Simon-Miller recalls the Shoemaker-Levy 9 impacts of
1994. "Before the comet hit Jupiter, it was torn
apart by Jupiter's gravity. As a result, we got to see
fragments of many different sizes make their marks in
Jupiter's atmosphere. Wesley's impact mark resembles
that of a mid-sized fragment a few hundred meters wide."